Added noDisk option that keeps each part's content in memory instead of in a temp file

Platform as a service offerings such as nodejitsu either forbid or limit disk writes. With the current implementation I was unable to large amounts of text (giant generated CSV files) to S3 as the application would throw errors.

I've added a ```noDisk``` option which when set to true keeps the data for each part in-memory instead of writing to a temporary file. It defaults to false.

Also added trivial change of including status code when an error is thrown because response code is not 200, as this was helpful for debugging.